Sound-Shadows in Water. Ill 



In confirmation of the foregoing view, the following obser- 

 vation may be cited : on the 16th of April, 1880, an explosion 

 of about 2000 or 3000 pounds of a nitro-glycerine compound 

 occurred at the " Giant Powder Works," situated under a 

 bluff on the eastern shore of the Bay of San Francisco, at a 

 distance (determined by triangulation) of 16,201 feet (4938 

 metres) in a direct hue, in a north-west direction, from my 

 room in the University building. About twenty -five adult 

 men, a majority of them Chinamen, were literally blown to 

 atoms, no one escaping to reveal the cause of the accident. 

 The concussion at the University buildings — more than three 

 miles distant — was sufficient to break about a dozen panes 

 of stout window-glass on the side next to the explosive centre. 

 Xearly every person about the University grounds experienced 

 two distinct shocks — one transmitted by the air, and the 

 other by the ground. The cottage occupied by my brother 

 (Professor Joseph LeConte) was situated in the geometrical 

 shadow of one of the buildings, being about 890 feet on the 

 further side of it. Xo aerial shock was experienced by him 

 or any member of the household : and the concussion trans- 

 mitted by the earth was alone felt as a shock emanating from 

 the floor. In other terms, the acoustical shadow cast by fcl 3 

 intervening structure completely cut off the sound-wave 

 coming by the air. It is scarcely necessary to add, that for 

 ordinary sounds such would not have been the result. 



20. Phenomenon of Jets. — The singular phenomenon ob- 

 served by my son (14), of numerous small jets projected 

 from the surface of the water when the shock transmitted by 

 the liquid reached the surface-area above the exploding 

 cartridge, was probably due to the circumstance that when the 

 short and intense elastic vrave emerged in a direction normal, 

 or nearly normal, to the aqueous surface, the tense superficial 

 capillary film yielded to the sudden impulse more readily at 

 some points than at others. The sensibly homogeneous 

 character of such a tensile elastic film would naturally tend to 

 group the points of rupture, or jets of water, into more or less 

 perfect order, partaking more or less of geometrical symmetry. 

 Hence the curious quincunx-like arrangement of jets as 

 viewed by the observer near the water-level. According to 

 this view, the phenomenon in question seems to find its 

 counterpart or analogue in the more or less symmetrical forms 

 produced by the intersection of the lines of rupture as the 

 result of the tensional strains due to the contraction of homo- 

 geneous masses, during the process of cooling or of desiccation. 

 Thus the columnar structure of certain igneous rocks seems 

 to be due to the tensile stress of contraction bv cooling after 

 K2 



